Silicone compositions have become widely accepted as release coatings, which are useful to provide a surface or material which is relatively nonadherent to other materials which would normally adhere closely thereto. Silicone release compositions may be used as coatings which release pressure sensitive adhesives for labels, decorative laminates, transfer tapes, etc. Silicone release coatings on paper, polyethylene, Mylar.RTM. and other such substrates are also useful to provide nonstick surfaces for food handling and industrial packaging.
Silicone release compositions are generally either heat-curable or UV-curable. The ultraviolet (UV) curable compositions have become more commercially acceptable for various applications due in part to the high energy costs and strict environmental regulations on solvent emissions which are attendant with the heatcurable compositions. Ultraviolet radiation is one of the most widely used types of radiation because of its low cost, ease of maintenance, and low potential hazard to industrial users. Typical curing times are much shorter, and heat-sensitive substrates can be safely coated and cured under UV radiation whereas thermal energy might damage the substrate.
Several UV-curable silicone systems are known in the art, including, Viventi, U.S. Pat. No. 3,816,282, Bokerman et al., U.S. Pat. No. 4,052,059 and Colquhoun et al., U.S. Pat. No. 4,070,526. These patents describe compositions where .omega.-mercaptoalkyl substituted polysiloxanes react with vinyl-functional siloxanes when exposed to UV radiation in the presence of certain photosensitizers.
UV-curable silicone resins with epoxy or acrylic functionality are also known in the art. These resins are found to have the degree of activity necessary to make them especially suitable for release applications. Illustrative of these compositions are Eckberg et al., U.S. Pat. No., 4,279,717; and Crivello et al., U.S. Pat. No. 4,026,705, as well as others.
It has been found that while these compositions give excellent stable aged release properties in most applications, there still exists room for improvement in employing the silicone release compositions on kraft paper and machine refined paper substrates. It would therefore represent a notable advance in the state of the art if a novel silicone release composition could be developed which provided further improvements of stable aged release properties on these substrates.
To this end, the present applicant has now found that if the substrate, such as kraft paper, is treated with a weak acid, improved aged release properties are obtained.